Fix lack of icon assignation when extension don't match capitalization (lowercase) (#3810)

- **PR Description**
The extension icon map contain all extensions on lowercase, when a file
don't have extension on lowercase the string don't match and icon is not
assigned.

Author: stefanhaller

pkg/gui/presentation/icons/file_icons.go

@@ -2,6 +2,7 @@ package icons
 
 import (
 	"path/filepath"
+	"strings"
 )
 
 // NOTE: Visit next links for inspiration:
@@ -728,7 +729,7 @@ func IconForFile(name string, isSubmodule bool, isLinkedWorktree bool, isDirecto
 		return icon
 	}
 
-	ext := filepath.Ext(name)
+	ext := strings.ToLower(filepath.Ext(name))
 	if icon, ok := extIconMap[ext]; ok {
 		return icon
 	}